Project Summary RNA molecules often must fold into distinct three-dimensional structures to exert their biological function. These folded structures may be large or small, long-lived or transient, and/or stable or unstable in nature. The kinetics of RNA folding is characterized by multiple pathways, the population of intermediates and often (but not always) on- and/or off-pathway kinetically trapped species. Our approach to understanding how the RNA is folded is to determine which folding pathways are possible in vitro with the goal of determining the subset that occur in vivo. We computationally integrate local and global measures of folding into `structural-kinetic' models that characterize folding reactions from their earliest steps. Our development of high-throughput methods for the acquisition of time progress curves with millisecond time and single nucleotide spatial resolution allows general hypotheses to be tested against experimental data that is both deep and broad. The proposed studies of group I introns seek to establish quantitative relationships between RNA structure, stability and folding kinetics by critically comparing the folding of phylogenetically related RNA molecules and gentle systematic perturbation of tertiary contacts. The folding of a riboswitch whose structure is homologous to the catalytic core of group I intron is analyzed to determine if these different regulatory elements possess a common folding mechanism. We explore the effect on the observed emergent folding behavior solution variables such as temperature and ionic conditions that affect the microscopic environment of RNA in order to understand the relationships between the physical environment and folding environment. Lastly, we seek to understand how transcription affects RNA folding.